Secondary batteries having a built-in protective circuit have been conventionally used for portable phones or portable personal computers. Thus, there are demands for safer protective circuits with increased charging capacity.
Reference 100 in FIG. 9 represents a battery device of the prior art comprising a chargeable/dischargeable accumulator 105, a switch element 104 and a control circuit 106.
The secondary battery device 100 further comprises a first and a second connection terminals 111, 112 between which can be connected an external circuit 110 consisting of a load or DC voltage source.
When the accumulator 105 has not been charged and an external circuit 110 consisting of a DC voltage source is connected between the first and second output terminals 111, 112, the accumulator 105 becomes charged by the external circuit 110.
Conversely, when the accumulator 105 has been charged and an external circuit 110 consisting of a load such as a portable computer is connected between the first and second output terminals 111, 112, a power is supplied to the external circuit 110 from the accumulator 105 because the first output terminal 111 supplies a positive voltage with respect to the second output terminal 112 that is at ground potential.
Reference U in FIG. 9 represents a protective circuit. This protective circuit U comprises a heater h and two fuses fa, fb as shown in FIG. 10.
The two fuses fa, fb are connected in series. The first output terminal 111 and the positive terminal of the accumulator 105 are connected to each other via the series circuit.
The heater h consists of a parallel circuit of two resistance heating elements ra, rb. One end of the heater h is connected to a junction P at which the two fuses fa, fb are connected to each other while the other end of the heater h is connected to the second output terminal 112 (and the negative terminal of the accumulator 105) via the switch element 104.
References ta, tb, tc in FIG. 10 represent terminals of the protective circuit U, among which references ta, tb represent both ends of the series circuit of the fuses fa, fb and correspond to a terminal connected to the positive terminal of the accumulator 105 and a terminal connected to the first output terminal 111, respectively.
Reference tc represents an end of the heater h and corresponds to a terminal connected to the switch element 104.
The switch element 104 is controlled by the control circuit 106. When a DC voltage source having an overrated output voltage is connected as the external circuit 110 between the first and second output terminals 111, 112, the control circuit 106 detects overvoltage between the first and second output terminals 111, 112 and turns on the switch element 104 so that the junction P is connected to the second output terminal 112 and the negative terminal of accumulator 105 via the heater h. As a result, a current supplied from the accumulator 105 and a current supplied from the external circuit 110 flow through the fuses fa, fb, respectively.
Both currents flow through the resistance heating elements ra, rb in the heater h, whereby they heat up.
The resistance heating elements ra, rb are located near the fuses fa, fb, respectively, and the fuses fa, fb are blown by the heat from the resistance heating elements ra, rb. The result is that both the current flowing from the external circuit 110 and the current discharged from the accumulator 105 are stopped.
If a short circuit occurs between the first and second output terminals 111, 112, however, the control circuit 106 does not operate and the switch element 104 remains off while the accumulator 105 becomes short-circuited at both ends and a short-circuit current is discharged from the accumulator 105.
The short-circuit current is very large because of the absence of resistance elements such as resistance heating elements ra, rb in the path through which the short-circuit current flows.
When the short-circuit current flows through the fuses fa, fb, at least one of them is blown by self-heating.
When one of the two fuses fa, fb connected in series is blown, the positive terminal of the accumulator 105 and the first output terminal 111 are disconnected and the short-circuit current stops.
The fuses fa, fb as described above must be blown by self-heating when a short-circuit current flows due to a short circuit between the first and second output terminals 111, 112, but they must not be blown when a normal load 110 is connected and a rated current flows.
However, there is a need to connect protective circuits U as described above in parallel to suit a wide range of rated currents is desirable.
Reference 101 in FIG. 11 represents a secondary battery device of a related art for the present invention, comprising multiple, for example, three protective circuits U1-U3 connected in parallel.
When a short circuit occurs between the first and second output terminals 111, 112 and a short-circuit current flows through the fuses fa, fb connected in series in each protective circuit U1-U3 in this layout, at least one of the fuses is blown.
However, it is unpredictable which of the two fuses fa, fb is blown, i.e., it cannot be determined whether the fuse fa on the side of the accumulator 105 or the fuse fb on the side of the first and second output terminals 111, 112 is blown.
FIG. 12 shows the fuses fa on the side of the accumulator 105 were blown in all the protective circuits U1-U3, and FIG. 13 shows the fuses fb on the side of the output terminal 111 were blown in all the protective circuits U1-U3.
If the fuses fa on the side of the accumulator 105 were blown in all the protective circuits U1-U3, as shown in FIG. 12, the positive output terminal of the accumulator 105 become completely disconnected so that the accumulator 105 stops discharging and the short-circuit current stops.
If the fuses fb on the side of the output terminal 111 were blown in all the protective circuits U1-U3, as shown in FIG. 13, the output terminal 111 is disconnected from all the protective circuits U1-U3 while the positive output terminal of the accumulator 105 is connected to only the switch element 104. In this case, the accumulator 105 stops discharging because the switch element 104 is not turned on.
As described above, the short-circuit current stops when all the counterparts of pairs of fuses fa, fb on either one side were blown in all the protective circuits U1-U3. However, if the fuse fb on the side of the output terminal 111 is blown in a protective circuit U1 and the fuse fa on the side of the accumulator 105 is blown in another protective circuit U2 as shown in FIG. 14, a residual current I101 continues to flow because the positive terminal of the accumulator 105 is connected to the output terminal 111 via the remaining fuse fa on the side of the accumulator 105, the remaining fuse fb on the side of the output terminal 111 and two heaters h.
This residual current I101 does not stop until the accumulator 105 completes discharging because this residual current I101 is a small current limited by the resistance values of two heaters h so that each heater h does not heat up enough to blow the remaining fuses fa, fb. Furthermore, the fuses fa, fb themselves do not heat up enough to be blown.
Table 1 below shows that the residual currents vary depending upon which of the two fuses, fa or fb, is broken. The residual current is zero when the fuses on the same side were broken, but in other cases, large residual currents of about 0.3 A flow.
TABLE 1Measurement resultsFuseHeaterBlownAfter fuse blowingresistanceresistancefuses *1BlowingResidualResistance valueNo(mΩ)(Ω)fafbtime (ms)current(A) *2between ta and tb (Ω)15.624.1◯X3500.00More than 1200 M5.621.2◯X(unmeasurable)5.021.6◯X25.422.9X◯3600.2933.35.122.2◯X5.621.1X◯35.722.6X◯3100.3032.45.622.9X◯5.221.0◯X*1: X . . . Blown  ◯ . . . Not blown*2: Current flowing between the output terminals after fuse blowing.
The various embodiments of the present invention were made to overcome the above disadvantages of the prior art with the purpose of providing a secondary battery device showing a small residual current after protective circuits have operated.